These vibrations, which are also referred to as juddering of the clutch, can be excited spontaneously or in a forced manner. Spontaneous clutch juddering is caused by a reduction in the frictional coefficient of the clutch lining in relation to the sliding speed, and may be of considerable severity depending on the overall damping of the drivetrain.
In contrast, forced clutch juddering is excited by sources that are external in relation to the friction partners, which may for example include axial vibrations of the crankshaft or deviations from parallelism at the clutch pressure plate in combination with an angular offset between the clutch pressure plate and the clutch disk that carries the friction lining.
The rotary vibrations in the drivetrain produced by clutch juddering are converted by the driven wheels into longitudinal vibrations of the vehicle as a whole and are transmitted by the operating elements and via the vehicle seats to the occupants of the vehicle. Clutch juddering is therefore perceived by the occupants as unpleasant vibrations or oscillations, which can also be associated with noise.
Although high damping in the components of the drivetrain reduces the vibration amplitude during forced clutch juddering, in view of the general desire for the lowest possible fuel consumption by the motor vehicle this is often an unrealistic requirement because permanent high damping in the drivetrain can essentially be achieved only by the continual elimination of friction losses, for example in the transmission, the bearings and the seals.
From EP 845 616 A2 it is known, in the case of a starting clutch or a converter bridging clutch, to detect juddering of the clutch by measurement means with the aid of suitable sensors and a control and regulating device, and adopt active measures to end this juddering. The measures consist in modifying the ignition timing of an internal combustion engine connected to drive the clutch, so as in that way to influence the input torque of the clutch. Another measure is to increase the contact pressure of the clutch pressure plate against the clutch friction lining, but this makes it no longer possible to operate the clutch in a slipping mode, with the many advantages of so doing.
From EP 1 539 526 B1 it is known that with the help of rotational speed sensors the speeds of the clutch input side and the clutch output side can be detected in order to recognize the presence of juddering vibrations. In this case the torque is not known, but it must be known in order to counteract the judder by a drivetrain excitation of opposite phase. Thus, an adaptation algorithm is used to modify the amplitude of the oppositely-phased torque excitation until the vibrations are damped. The disadvantage of this is that to determine the oppositely-phased vibration amplitude the adaptation algorithm requires a certain recognition time. For this, the torque fluctuations have to be continually determined by the adaptation algorithm since the conditions (speed, torque, temperature, etc.) in the drivetrain can vary continually so that the fluctuation amplitude does not remain constant. Owing to the time required for amplitude recognition, damping or elimination are insufficient.